The invention relates to a circuit device for operating a discharge lamp by means of a high-frequency current, which circuit device comprises input terminals for connecting it to a low-frequency supply voltage source, rectifier means coupled to the input terminals for rectifying the low-frequency supply voltage, a first circuit coupled to a first output terminal N3 of the rectifier means and to a second output terminal N5 of the rectifier means, which first circuit comprises a series arrangement of first unidirectional means, second unidirectional means and first capacitive means, which circuit device further comprises inverter means connected in parallel with the first capacitive means for generating the high-frequency current, a load circuit comprising a series arrangement of inductive means, second capacitive means and means for applying a voltage to the discharge lamp, which load circuit couples a terminal N1 of the inverter means to a terminal N2 between the first unidirectional means and the second unidirectional means, and a second circuit coupling a terminal N2 to a terminal N4 and comprising third capacitive means, a third circuit coupling the first output terminal N3 of the rectifier means to a terminal N4 between the second unidirectional means and the first capacitive means, which third circuit comprises a series arrangement of third unidirectional means and fourth unidirectional means, neither said first circuit nor said third circuit comprising inductive means, and a fourth circuit coupling the third circuit to a terminal N6, which forms part of the load circuit.
Such a circuit device is known from WO 97/19578.
The known circuit device has an optimum operating point at a load voltage which is approximately half the low-frequency supply voltage. In the case of sub-optimal discharge lamps, the arc voltage must be adapted. For this purpose, use can be made of a capacitive voltage divider; in addition to a capacitor coupling the terminal N4 to a terminal N7 between the third unidirectional means and the fourth unidirectional means, a capacitor coupling together the terminals N4 and N7. The use of capacitive voltage division, however, causes high currents in the inductive means of the load circuit and in the inverter means, particularly the switching elements of the latter. This is problematic, in particular, in the case of circuits having an electric power in excess of 100 W, since it requires additional cooling of the switching elements.
It is an object of the invention to provide a more efficient method of adapting the power feedback source to the low-frequency supply voltage.
To achieve this, the invention provides a circuit device of the type mentioned in the opening paragraph, which is characterized in that the circuit device comprises a fifth circuit by means of which the fourth circuit couples the third circuit to a terminal N6, said fifth circuit comprising transformer means which couple the terminal N4 to the fourth circuit, and a branch of which is coupled to a terminal N7 between the third unidirectional means and the fourth unidirectional means.
In accordance with the present invention, preferably the fifth circuit comprises fifth capacitive means coupling the branch of the transformer means to terminal N7 to make sure that a direct current leading to saturation of the transformer means does not flow through the transformer means and the fourth unidirectional means.